Update login-handler.md

Cast return type from getRedirect in LoginHandler example to string, referenced by #17

Signed-off-by: sheridans <[email protected]>

docs/book/v1/login-handler.md

@@ -140,7 +140,7 @@ class LoginHandler implements RequestHandlerInterface
             }
         }
 
-        return $redirect;
+        return (string)$redirect;
     }
 
     private function handleLoginAttempt(